Having Single Channel RAM is the budget option for most. But then you’re missing out on the speed of Dual Channel RAM, which doubles the bandwidth that can flow through both RAM sticks. The difference can be observed in real-time processes that require encoding and decoding, like watching a high quality video, gaming, previewing a production in animation or video editing software, developing a game and so on.
If you are to use Single Channel RAM, you will experience hiccups and random stutters at random interval during use of software.
In the case where you have Single Channel RAM, and want to add Dual Channel to the setup, you must remove the Single Channel RAM, otherwise the computer will count the new memory sticks as Single Channel. Just sell your Single Channel RAM in such a scenario.
Also if you do not get a full 4 stick set, you can always get the other 2 down the line, but you should keep to the same brand and same parameters, otherwise you will suffer gravely on the path ahead.
Preferred amounts of RAM for general computer use is 16GB, 32GB is for advanced production for animation, modelling, game design. 64GB is for professional use-case. For instance Unreal Engine 5 advises 32-64GB for development on their engine. 8GB is forbidden for use with Windows, but can work nicely on Debian Linux.
What does more RAM provide? It gives you more room to do multitude of processes, having more software open and running at the same time, which creates the convenience of swiftly switching between them.
But remember that even if you have a ton of RAM, you can be bottlenecked by your processor not being able to process all those tasks at once. So it’s important to balance that out by acquiring a powerful enough CPU that can compliment the amount of RAM.